Integration of optical source devices, optical modulating devices and photo-detecting devices on a single substrate to produce a photonic circuit has become an area of intense research in recent years as there are numerous advantages to such monolithic integration such as compact size, reduced weight and cost, and rugged construction of the photonic circuit.
Typically, waveguides are used to route the light to and through the various devices residing on the substrate. Recently waveguides incorporating quantum wells have been used as modulators and detectors by taking advantage of the occurrence of the large red shift of the absorption edge in the quantum wells when electrical field is applied. This red shift is referred to as the Quantum Confined Stark Effect (QCSE). For example, a GaAs quantum well under the influence of an applied field normal to the layers of the quantum wells will exhibit a large shift in the absorption edge due to the QCSE. The QCSE allows a waveguide that contains quantum wells to be used as an optical modulator or a detector.
The lithographic fabrication process for an integrated photonic circuit is much the same as the process for integrated electronic circuit except for the significant technical difficulty in integrating an optical modulator and a laser diode as optical source. The difficulty lies in the fact that laser sources require the inclusion of an epitaxial structure different from the rest of the circuit. The reason is that the optical modulator requires a semiconductor having a band gap energy that is different from that of the laser diode. The extant solution is to accomplish the monolithic integration of the laser diode through a multiple layer design in which the laser source is not co-planar with the other devices. Alternatively, etch-and-regrowth process can be performed which is costly and complex, requiring at least 12 steps.
In the monolithic integrated photonic circuit, light travels through multiple quantum well channel waveguides and is coupled into and out of the co-planar devices which reside on a single semiconductor substrate. Each of the devices is comprised of a quantum well channel waveguide of a pre-determined length and an electrical contact pad mounted on the waveguide, the contact pad facilitating the application of electric field to the device. The function of any particular device as an optical source, an optical modulator or a photo-detector is determined by the bias mode of electric field that is applied normal to the layers of that particular device.
The integrated photonic circuit is fabricated by growing and doping a semiconductor chip so that it is made up of layers that together exhibit a positive-intrinsic-negative doping profile where multiple quantum wells are contained in the middle layers. After placing the contact pads on the chip in a pre-arranged fashion, the chip is etched to a pre-determined depth, thereby producing a photonic circuit with raised waveguides with gaps between the waveguides for electrical isolation.